1. Field
The present disclosure relates to power generating wind turbines and more particularly pertains to a wind turbine blade and assembly having a greater ability to capture energy from wind than known designs.
2. Description of the Prior Art
Windmills have long been used to extract kinetic energy from the wind. More recently wind turbines have been designed to harvest the kinetic energy from the wind and convert this kinetic energy into electrical energy. The preferred type of wind turbine for electricity generation applications is a horizontal axis wind turbine (HAWT). A horizontal axis wind turbine is a wind turbine whose rotor hub axis is mounted substantially horizontally with respect to the ground.
Over the years improvements have been made to earlier wind turbine blades. The focus of these improvements has been primarily directed toward airfoil designs. In using airfoil technology, improvements have focused on a number of areas of airfoil design—the shape of the airfoil and the pitch angle θ of the airfoil are two important factors. The shape of these foils has always required a high Reynolds number.
In the efforts to discover a better shape to achieve greater lift, and thus increased efficiency, the improvements have primarily focused on the Bernoulli principal of aircraft wing design that holds that the major lift on an aircraft wing is caused by the presence of a relatively reduced naturally existing ambient air pressure on the upper surface of the wing as it travels through the moving wind. A review of current literature indicates that scant attention has been paid to the increased air (lift) forces on the lower surface of the airfoil. The lift generated by the airfoils of conventional wind turbines is translated into rotational torque. The developments in wind turbines have included the use of very long, streamlined airfoils for the blades. However, blades with a long length have tip speeds that are extremely high (above 170 mph), and thus the leading edge speed of the airfoil moving through the air varies significantly along the length of the blade. This factor has lead to the pursuit of a better twist characteristic for the airfoil design to match the different speeds of the airfoil at different distances from the root of the blade.
Other improvements include systems to rotate the blade about the longitudinal axis of the blade to dynamically vary the pitch angles of the airfoil in an attempt to avoid stall conditions for the airfoil. Continuous monitoring of the wind speed and the pitch angle θ of the airfoils permits the pitch angle θ to be continuously varied in an attempt to match the pitch angle θ to the wind speed and thereby avoid stalling as well as increasing the lift of the airfoil and thus maximize the kinetic energy extracted from the wind. Stalling is a condition where the airfoil loses lift due to excessive pitch relative to the wind speed, and as a consequence the airfoil is unable to exert torque. The range of effective pitch angles for the airfoil designs that are commonly employed is approximately 10 degrees to approximately 17 degrees. Pitch angles greater than this range typically result in stalling of the conventional airfoil designs. This limited range of effective pitch angles limits power output.
The first purpose of variable pitch systems is to control the revolutions per minute (RPMs) of the electrical generator. An alternating current generator must turn at the exact revolutions of the cycles of the alternating current in the electrical grid into which the electricity is being utilized. Off cycle electricity is useless and harmful to the system. Further, variable pitch systems are complicated, expensive, high maintenance and there failure is very costly.
Typically, the blades of the wind turbines employ airfoils designed to have high Reynolds numbers. Airfoils with high Reynolds numbers typically have a sleek shape that moves through the air at a high rate of speed while offering a minimum amount of drag or resistance to the rotation of the blade about the substantially horizontal axis of the wind turbine. Common airfoil designs employ only a small degree of camber, which is the ratio of the difference of the distance between the chord line and the mean camber line (at any point along the chord line) divided by the length of the chord line. Some have a camber value of less than 4% camber, but camber values above approximately less than about 4% are considered unsuitable for use with wind turbines as airfoil designs with such camber values have unacceptably low Reynolds numbers for current designs, which experience blade tip speeds of high velocity.
The design of airfoils is and has been based on airfoils in flight such as airplane wings. Because the application of Bernoulli's principals dominates the design of this technology, the shape of the lower surface of the airfoil has received scant attention in airfoils designed for wind turbines. The lower surface of many airfoils used on wind turbines today are influenced by the desire to strengthen the support structure for the blade by creating a deeper cross section to resist bending or breaking of the blade. The desire to have a deeper cross sectional area thus frequently influences the shape of the lower surface of these airfoils. The great lengths of the blades typically utilized on current wind turbines thus requires a strengthened structure which in turn affects the shape of the lower surface of the airfoil. Because of the focus on the Bernoulli principal and the structural design constraints, the lower surface of wind turbine airfoils is greatly underutilized as a harvester of kinetic energy.
Yet Newton's Laws teach that there is a great potential for harvesting energy with the lower surface of an airfoil. However, the lower surfaces of current wind turbine airfoils are not designed to maximize the harvesting of the winds kinetic energy efficiently.
Furthermore, it has been found that the airfoil designs of most existing wind turbines have a relatively high cut in wind speed, which is the lowest speed at which the force of the wind acting on the airfoil overcomes factors such as starting friction or inertia and begins producing usable power. Typically, the cut-in wind speed is about 8 miles per hour or higher, which means that wind speeds lower than about 8 miles per hour do not result in power generation.
In these respects, the wind turbine blade design according to the present disclosure substantially departs from the conventional concepts and designs, and in so doing provides a wind turbine blade and assembly believed to be more effective at capturing energy from the wind than the conventional concepts and designs.